1. Field of the Invention
The invention concerns a method for supplying electrical power to an automobile headlight lamp so that the lamp has both sufficient luminous intensity and a sufficient service life to be installed in an automobile vehicle. The invention also concerns an automobile vehicle headlight in which this method is used.
The invention finds applications in the field of automobile vehicle lighting and in particular in the field of supplying electrical power to lamps in vehicle headlights.
2. Description of the Related Art
In the field of automobile lighting, there exist various types of light projection devices, essentially including:                side lights, of low intensity and range;        dipped beam headlights of higher intensity and having a range of about 70 meters on the road, used essentially at night and where the distribution of the light beam is designed not to dazzle the driver of an oncoming vehicle;        main beam headlights, with a long range of around 200 meters, which are used to illuminate the road far ahead and to enable the driver to see clearly where the vehicle is headed;        fog lamps.        
At present, there exist light projection devices, or headlights, which provide both the dipped beam headlight function and main beam headlight function. These headlights are called dual-function or dual-mode headlights. They alternate between dipped beam headlight mode and main beam headlight mode. This kind of dual-function headlight has a light source emitting exactly the same light in both modes of operation (dipped beam or main beam). It generally includes a removable cap adapted to intercept the light beam. For example, this cap is a metal shutter that can assume either a first position or a second position. In the first position, the cap does not block the light beam produced by the light source, or lamp, of the headlight. In the second position, the cap partially blocks the light beam produced by the lamp of the headlight. In this second position, the cap produces what is referred to as a specific cut-off of the light beam, and this specific cut-off corresponds to the cut-off of the light beam necessary to obtain the dipped beam function. Thus the dipped beam mode and the main beam mode differ in the presence or absence of the cut-off.
In most dual-function headlights, the removable cap pivots. It can thus assume the aforementioned two positions, namely:                the second position, called the blocking position, in which it intercepts the light beam, thereby generating the dipped beam function, and        the first position, called the neutral position, in which the cap does not block the light beam, thereby generating the main beam function.        
FIG. 1 shows one example of an automobile vehicle headlight. This headlight 10 generally includes a casing 1 closed by a protective glass 2. The headlight 10 includes, inside the casing 1, a lighting module 3, for example an elliptical module. The lighting module 3 includes, in particular, a light source 4, also called a lamp, a reflector 5 adapted to reflect light toward the protective glass 2, a lens 6 adapted to diffuse the light, a removable cap 7 and power supply means 8 for the light source 4.
In present day dual-function headlights, the light source can be a halogen light source. This halogen light source is generally supplied with power at a fixed voltage, which depends on the light source used, the vehicle to which the light source is fitted and/or the country in which the vehicle is to be sold. This fixed voltage is generally the nominal voltage of the light source. A nominal voltage is specified by lamp manufacturers for each type of light source. This nominal voltage is the power supply voltage at which the light source functions optimally.
To enable the headlight to illuminate correctly the road in front of the vehicle, a light source must offer good performance, i.e. high power. The performance of a light source is characterized primarily by its luminous flux and its luminous intensity. Its performance, luminous flux and luminous intensity depend on the voltage at the terminals of the light source. The higher the voltage at the terminals of the light source, the higher the luminous flux and the luminous intensity. For example, considering the luminous intensity: for a standard H9 type lamp, the luminous intensity is of the order of 3400 cd/cm2 for a nominal voltage at the terminals of the lamp of 13.2 Volts and 3150 cd/cm2 for a voltage of 12.8 Volts.
Now, the person skilled in the art knows that the higher the voltage at the terminals of the lamp, the shorter the service life of the lamp. In other words, increasing luminous intensity is achieved to the detriment of the service life of the lamp. For example, with a voltage of 11.78 Volts, an H9 lamp has a service life approximately 4.4 times greater than that corresponding to its nominal voltage of 13.2 Volts, which in this case is of the order of 250 hours.
It is therefore difficult at present to arrive at a compromise between performance and service life. Automobile manufacturers must choose, for equipping a vehicle, between a lamp with a long service life and a lamp with a high luminous intensity. They must therefore choose between two lamp types:                either a lamp offering a long service life, such as an H7 Long Life lamp, which offers moderate performance, with a luminous intensity of the order of 2350 cd/cm2 and a long service life (of the order of 700 hours). This kind of lamp has the drawback of offering relatively low power illumination, essentially in the main beam mode where luminous intensity is very important,or        a lamp offering very high performance, such as an H9 lamp, which offers very good luminous intensity (of the order of 3400 cd/cm2) but a short service life (of the order of 250 hours). This type of lamp has the drawback that it must be changed frequently, which leads of course to an additional cost for the owner of the vehicle. The service life of this kind of lamp can be increased by reducing the voltage at the terminals of the lamp. With a power supply voltage of 11.78 Volts, for example, the service life of the lamp is approximately 1095 hours, although its luminous intensity falls to 2350 cd/cm2, which corresponds to the luminous intensity of an H7 Long Life lamp. This kind of lamp with a reduced power supply voltage therefore offers the same performance in terms of power as an H7 Long Life lamp.        
What is needed, therefore, is a system and method that overcomes one or more of the problems of the prior art.